What is Compressed Air?

Air at greater than atmospheric pressure (usually 100 pounds over atmospheric pressure)
or 100 PSIG.

P.S.I.G. means Pounds Per Square Inch Gauge.

Atmospheric air can be compressed in several ways:

POSITIVE DISPLACEMENT COMPRESSORS

are machines in which successive volumes of air are confined within a closed space and elevated to a higher pressure.

DYNAMIC COMPRESSORS

are machines in which air is compressed by the mechanical action of rotating impellers imparting velocity and pressure into the air.

CENTRIFUGAL COMPRESSOR

operates like a fan.